The proposed studies will attempt to test the hypothesis that the differentiation of medial edge cells of the developing secondary palate is instructed by the subjacent ectomesenchyme. The differentiation of the medial edge cells, which involves their eventual death, will be assayed by several methods including the uptake of neutral red or acridine orange, morphological changes in epithelial cells observed by scanning electron microscopy (SEM), or the localized binding of concanavalin A to the medial edge cells. Four specific studies will be undertaken. The in vitro occurrence of epithelial-mesenchymal interactions will be studied by making isochronic, heterochronic, homologous, and heterologous tissue recombinants between tissues derived from the secondary palate of the alligator and mouse, which undergo fusion, and quail and chick, which do not fuse. It is expected that epithelial behavior will be dictated by the associated mesenchyme. In order to understand the physical relationship between the medial edge epithelium and the associated mesenchyme, the temporal and spatial changes in the intervening basal lamina will be examined in vivo and in vitro by indirect immunofluorescence for fibronectin, type IV collagen, laminin, and basement membrane proteoglycan and by light microscopy (LM) and transmission electron microscopy (TEM). The mechanism of epithelial-mesenchymal interaction will be studied in vitro. The requirements for cell contact will be examined in transfilter experiments. The relative inductive abilities of cells from different regions of the secondary palate will be compared; the time of required interaction will be determined; and whether the influence is present in the basal lamina, mesenchyme cytoplasm, or nuclei will be studied. Lastly, the ability of premigratory neural crest to provide the stimulus will be examined. Interspecific recombinants will be used in all these approaches.